A hot runner is utilized to transfer molten material, typically plastic resin, from an injection molding machine to a mold. A hot runner generally includes a manifold plate, a manifold housed in the manifold plate, and a backing plate that encloses the manifold in the manifold plate. The manifold, typically heated via a plurality of tubular heaters embedded therein, routes molten resin from a sprue bushing, which mates with an injection unit on an injection molding machine, to a plurality of nozzles which inject the molten resin into cavities in the mold. The manifold divides the flow of the molten resin into a network of a plurality of melt channels as it flows from the sprue bushing to the nozzles, all the while maintaining a near constant temperature of the resin throughout.
The state of the art includes various nozzles and nozzle tips for a hot runner which is typically of either a valve gate style or a hot tip style. In the valve gate style, a valve stem reciprocates within the nozzle, nozzle tip and a gate orifice acting as a valve to selectively preclude or allow the flow of resin through the nozzle tip and into a mold cavity. In the hot tip style, a small volume of resin at the end of the nozzle tip, in the gate orifice, solidifies during each molding cycle thus precluding the flow of resin into the mold cavity. The present invention describes the hot tip style nozzle.
It is important to note that the nozzle tip is subject to many influences which help determine its size and makeup. The nozzle tip must be able to withstand loads from injection pressures that may reach 40,000 psi (275 MPa) or more, endure corrosion and chemical attack, and resist abrasion and wear from resins filled with glass or other particulate materials. Paramount to the nozzle tip is its ability to provide the correct amount of heat to the gate orifice to allow sufficient flow of resin to the mold cavity yet promote solidification of the resin once the mold cavity is filled. To enable this feature, a heater is installed to encircle the nozzle in an area proximate to the nozzle tip, and the nozzle tip is typically constructed of a highly thermally conductive alloy, usually a copper alloy, which, by nature, tends to be relatively low in hardness. All these factors contribute to the nozzle tip eventually wearing out or failing thus necessitating its replacement, generally more frequently than most other components usually replaced during regular, periodic maintenance of the hot runner. For this reason, it is desirable to be able to service the nozzle and the nozzle tip in a quick and efficient manner without necessarily disassembling the entire hot runner or even removing and re-wiring the heater.
A common, and simple, nozzle housing and nozzle tip configuration involves a nozzle tip, having a male thread, being installed into a nozzle housing which has a female thread. The nozzle housing, typically made of a high-hardness tool steel, extends over the nozzle tip, beyond the threaded connection, to include, at its end, a thin, raised band of material; a seal ring, configured to fit diametrically inside a similarly sized bore in a gate insert within a mold, with some clearance at room temperature, such that at operating temperature, its radial, thermal expansion creates a gate seal therebetween to preclude molten resin from leaking between the seal ring and the gate insert.
When the mold, and consequently, the gate insert, is removed from the hot runner during maintenance or product changeover, the seal ring of the nozzle housing is disengaged from the bore of the gate insert. Though there is a nominal clearance between the two surfaces at room temperature, if disassembly is performed before the nozzle housing has cooled sufficiently from its operating temperature to reduce its radial, thermally-expanded diameter, or if the two surfaces are slightly misaligned, the result will be abrasion of the two mating surfaces. Any slight scratches or abrasion of the seal ring on the nozzle housing may potentially provide a path for pressurized, molten resin to leak by, during operation, resulting in catastrophic damage to the hot runner. Over time, this abrasion will require replacement of the entire nozzle housing to prevent, or repair from, resin leakage, thus necessitating significant down time of the hot runner for its maintenance as the entire hot runner must be disassembled in order to remove the nozzle housing from between the manifold plate and the manifold.
The thin section of the seal ring of the nozzle housing is also its weakest point, and is subjected to the same high injection pressures as the nozzle tip. The trend of the injection molding industry to reduce the cost of a molded part by reducing the amount of resin required to fill it, necessitates a thinner molded part wall thickness thus requiring higher injection pressures. To utilize stronger materials to make the seal ring of the nozzle housing more robust, is cost prohibitive as the seal ring and the entire length of the nozzle housing and are integral.
To address these needs and concerns, a two piece tip assembly is commonly utilized, as is illustrated in U.S. Pat. No. 6,609,902 B1 to Blais et al, for example. A removable tip insert is secured against a nozzle by a tip retainer which is typically threadably connected to the nozzle, whereby a flange of the tip insert is trapped by a mating shoulder of the tip retainer. The tip retainer also has the added feature of having the seal ring included at its gate end. The relatively inexpensive tip insert can be removed and replaced by unscrewing the tip retainer, installing a new tip insert, and re-attaching the tip retainer. Such a tip arrangement is cost effective in that the tip retainer is not discarded.
However, this two piece design is not without its limitations. In order to create sufficient seal force, the flange of the tip insert is subjected a high torque load by the retainer, creating a stress concentration at the corner of the flange and the tip insert. When subjected to resin at operating temperature and pressure, the tip insert is prone to cracking and failing at the base of the flange. Additionally, the cumulative design of the flange and retainer assembly imposes restrictive size limitations on the diameters of the components thereby limiting the injection pressures and loads they may withstand.
For the foregoing reasons, the present invention is directed to overcoming one or more of the problems or disadvantages set forth above, and for providing a hot runner nozzle system with replaceable componentry capable of withstanding high injection pressures.